CD4 is a cell-surface glycoprotein found on the mature helper T cells and immature thymocytes,as well as on monocytes and macrophages. (Some cytotoxic T cells have CD4 protein as well.) Normally,about 65% of T cells in the blood are CD4+ (have CD4 protein protruding from their membrane). A mature T cell with either have CD4 or CD8,but not both. During one stage of development T cells develop CD4 and CD8 receptors,but they eventually are differentiated in the thymus and become more specialized.